The invention relates generally to Schottky-barrier semiconductor devices, and uses therefor. More particularly, the invention is directed to Schottky-barrier semiconductor devices for use in measuring strain, temperature and the like.
Methods are known in the art for measuring mechanical strain, which are based on measurements of resistance, the piezoelectric effect, or the acousto-optical effect. Conventional strain gauges vary in speed, sensitivity, bandwidth, system complexity and cost.
Conventional resistive strain gauges have a conductive element, such as a metal ribbon, epoxied to a flexible member. The gauge is included as a component in a balanced Wheatstone bridge resistive network. The strain produced by vibrations transfers through the flexible member to the metal ribbon and alters its resistivity. This fractional change in the resistivity unbalances the bridge producing a current proportional to the strain. The resistive strain gauge is a narrow band device, responding only in a limited bandwidth in the vibrational spectrum. Dynamic calibration of the bridge is a tedious process and integration in sliding contactsxe2x80x94in which it is desirable to measure strainxe2x80x94requires careful packaging.
Some of the disadvantages of the resistive strain gauge are overcome by using a voltage or capacitive method to measure strain. Certain crystals such as quartz and PZT are piezoelectric in nature. Under stress or strain they become polarized, and the polarization appears as a voltage across the crystal, or alternatively as a capacitive charge. The piezoelectric transducer is packaged in a metal fixture that is mated to the vibrating assembly. The voltage produced by the transducer is amplified electronically. However, sensitive piezoelectric crystals are expensive to fabricate, package, and calibrate. Furthermore, each crystal requires its own calibration.
Strain coupled to an acousto-optic material such as quartz, LiNbO3, or water produces changes in the refractive index of the material. Laser light incident on the material can undergo Bragg diffraction or deflection. The angle of deflection is proportional to the vibrating stimulus, and the deflection is measured on a position-sensitive photodetector array. This is a sensitive method of detecting high frequency vibrations. However, high quality crystals for acoustic sensing are expensive, and the experimental apparatus required for a simple measurement is fairly complex, and unsuitable for measuring strain at multiple locations in a commercial engine, hydraulic system, transmission or the like.
Methods are known in the art for measuring temperature. A common technique used for electrical temperature sensing is to measure the motion of charges in a sensor element. Temperature sensors are commonly constructed as resistors, bi-metal junctions, or semiconductor devices, and require a variety of measurement and calibration techniques.
In resistive temperature sensors, the mean-free path of the electrons between scatter events is related to the temperature. As the temperature increases, the vibratory motion of ions in the resistive material increases, and electrons traveling through the resistive material have a higher scattering probability. The electron mean-free path is reduced and the sensor becomes more resistive. When the temperature declines, the inverse process is true. Resistive sensors often have carbon-glass or metal-film resistors. For example, if aluminum is used as the metal film sensor material, its resistivity changes from 3.55 xcexcxcexa9-cm at 300xc2x0 K. to 2.45 xcexcxcexa9-cm at 273xc2x0 K. and 0.3 xcexcxcexa9-cm at 77xc2x0 K..
Bi-metal junctions are formed when two dissimilar metals make physical contact. A potential barrier is formed at the junctions. Electrons are thermally excited over the barrier, and the current across the barrier is temperature sensitive. Bi-metal thermocouples are most useful at high temperatures.
Semiconductor temperature sensors are usually constructed as p-n junction diodes, bipolar junction or field-effect transistors. The motion of electrons through a crystal is temperature dependent. The crystalline lattice is in a state of vibratory motion in well-defined phonon modes. The phonons scatter electrons randomly. If the temperature of the material is comparable to its Debye temperature, the mode density, governed by Bose-Einstein statistics, is fairly high. The probability of an electron-phonon scatter event is proportionally high, and the electron""s mean-free path is relatively short. As the sensor temperature decreases the motion of the crystalline lattice xe2x80x9cfreezes.xe2x80x9d Fewer phonon modes are excited in the lattice, and the electron motion is relatively unhindered by any interaction with the lattice. Electrons, may however, undergo Coulomb scattering by a sparse population of impurity ions until the carrier motion itself freezes. This occurs at very low temperatures approaching a few degrees Kelvin.
An alternative technique for temperature sensing in semiconductors is to simply monitor the thermionic emission across the potential barrier in a p-n or a metal-semiconductor junction. The emission probability is proportional to the classical Maxwell-Boltzmann distribution. Sensors using this alternative technique are particularly effective at high temperatures where thermionic emission increases exponentially.
Any of the above-described temperature sensors may be used in a conventional temperature-sensing system as a part of a balanced resistive network powered by a constant-current source. The temperature is measured by monitoring the voltage across a reference resistor held at a fixed temperature. The temperature dependent current through the sensor is deduced from this voltage.
It is known in the art of semiconductor technology that when a semiconductor is brought into contact with a metal, a barrier layer is formed in the semiconductor from which charge carriers are severely depleted. This barrier is known as a Schottky barrier. a Schottky diode is formed in the region where a metal contacts a lightly doped semiconductor. Schottky diodes have a faster response time and lower operating voltage than doped silicon junction diodes. Metal in contact with a highly doped semiconductor (5xc3x971017 atoms per cubic centimeter), however, forms a regular ohmic contact. The Schottky barrier forms because the work function of the metal is greater than the work function of the doped semiconductor, and the metal depletes the semiconductor in the region around the contact of charge carriers, typically electrons, leaving in the semiconductor a depletion layer of positively charged donor ions that is practically stripped of electrons.
Semiconductor devices using the Schottky barrier are known in the art. Scares, S. F., xe2x80x9cPhotoconductive Gain in a Schottky Barrier Photodiodexe2x80x9d, Jpn. J. Appl. Phys., Vol. 31, pp. 210-216 (1992), discloses a pair of metal contacts on a lightly-doped n-type semiconductor for use in photodetection. At each interface between metal and semiconductor, a Schottky barrier forms. In xe2x80x9cHeterodyne Ultraviolet Photodetectionxe2x80x9d (Dissertation of S. F. Scares, University of New Mexico, Albuquerque, submitted December, 1989), metal contacts are deposited about 3 microns apart on a lightly-doped silicon substrate, and each has an area in contact with the substrate of about 50 square microns to about 250 square microns. The semiconductor dopant density is selected so that the depletion regions of the two contacts almost extend to each other. It is further known that application of a bias across the pair of Schottky barriers enhances the detection of photo events.
There is a need for a simple, inexpensive, fast, robust strain gauge for inclusion in gears and bearings in aircraft engines and transmissions, for example, and engines and transmissions generally. Resistive gauges are difficult to package and time consuming to calibrate. Piezoelectric crystals are expensive and require complex electronic circuits to detect minute stress-induced voltages. Acousto-optic techniques are based on expensive laser techniques.
There is a further need for a temperature gauge for these same engine environments, especially a gauge that can cooperate with a strain gauge, where strain measurements may be thrown off by changes in temperature due to friction or combustion heat from operation.
The invention is embodied in a novel and useful device for measuring physical variables such as mechanical strain and ambient temperature by directly or indirectly measuring changes which these physical variables impart to Schottky-type electrical barriers in the device.
The device comprises a flexible, lightly-doped semiconductor substrate in the form of a leaf, with at least one metal contact formed on the upper surface thereof, forming a Schottky barrier at the interface of the metal contact and the substrate. More specifically, the device has a pair of metal contacts formed on the upper surface of the substrate for connection to current or voltage measuring equipment, each metal contact forming a Schottky-barrier diode. The Schottky-barrier electrical potential at a metal contact is altered by strain in the substrate leaf, and this electrical potential change is registered in this invention as an indication of that strain. In this manner, the device is a strain gauge.
The electrical potential change can be registered by measuring the current flowing through the Schottky barrier at a metal contact under an external electrical bias. Alternatively, the electrical potential change can be registered by measuring the voltage across the Schottky barrier at a metal contact.
The substrate is preferably formed as a leaf, e.g., sufficiently thin from its upper surface to its lower surface to be flexible. Furthermore, at least one metal contact must interface with the substrate over a sufficiently large area of its upper surface to provide a measurable current or voltage signal indicative of the strain, or of some other physical variable change such as a change of temperature.
The inventive gauge is small, simple, and fast-to-respond. It does not require extensive calibration. It does not require expensive equipment for measuring a response to strain. The range of the device is superior to devices in the art.
The device can be epoxied at its lower surface to an engine part, transmission gear, or any other component, the strain in which is to be measured. Alternatively, the device can be formed by vapor deposition directly on the underlying engine part during fabrication of the part.
Under strain transmitted from the strained engine part, through the epoxy and into the semiconductor substrate, the dopant atoms are redistributed in the semiconductor lattice. The dielectric tensor of the semiconductor substrate is also altered by the strain, producing a bulk polarization across the semiconductor lattice. The Schottky-barrier electrical potential varies as a function of the distribution of the dopant atoms and as a function of the dielectric tensor, so that the changes of these two factors caused by strain likewise causes a change in the Schottky-barrier electrical potential.
The change in electrical potential can be measured directly with voltage measuring equipment connected to the metal contact or contacts, as an indication of the strain. As a preferable alternative, an electrical bias is applied across the Schottky barrier at a metal contact, and the current flow across the barrier is monitored. As the Schottky-barrier electrical potential raises or lowers with strain, the current flow across the barrier is reduced or increased, respectively, as an indication of the strain.
Importantly, the device must be thin enough that strain is adequately imparted from the underlying engine part, for example, through the substrate lattice to the region at the upper surface of the substrate where the Schottky barriers are found. Thinness is also critical because the thinner a device is, the more freely it flexes with the underlying member, without exceeding its elastic limit and fracturing.
The device is formed by a sequence of known semiconductor fabrication techniques. A lightly-doped semiconductor substrate may be directly deposited on an underlying engine part by means of vapor deposition, or a lightly-doped semiconductor substrate may be provided as a prefabricated wafer. At least one metal layer is deposited on a doped semiconductor substrate over an area of sufficient size to provide a measurable change in the Schottky barrier with strain in the substrate. The metal layer is about 30 nanometers thick, and preferably about 5,000 to about 10,000 square microns in area. Preferably, two such metal contacts are formed on the upper surface of the substrate at distinct locations about 1-100 microns apart across the semiconductor substrate surface. For measuring strain, the substrate is thinned down, or deposited, to a thickness of about 10 microns. Additional metal layers may be deposited on the initial metal contact or contacts to provide electrical connectivity to other equipment. Many devices may be created on a single prefabricated wafer, and subsequently cut apart into individual devices.
The overall size of the gauge is about 100 to about 200 microns on a side, making it a very small device, capable of being attached to or embedded in a variety of surfaces in engine, hydraulic or motor components. It can be attached to a surface with an appropriate epoxy. Each of the metal contacts is connected to electrically-conductive leads which permit direct or indirect measurement of the Schottky-barrier potential by measuring equipment, such as ammeters or voltmeters.
Orientation of the gauge with respect to the desirably-measured strain is important in two respects. First, the orientation of the crystal lattice of the semiconductor substrate with respect to the direction of strain is important. Greater sensitivity to a given stress can be obtained in the device if the device is produced and used in a manner such that the anticipated stress occurs in the direction of greatest packing density in the semiconductor lattice of the leaf. Second, the orientation of the two contacts with respect to the crystal orientation is important. It is preferred that a line drawn between the metal contacts is parallel to the direction of greatest packing density in the semiconductor lattice of the leaf.
The device of the present invention can also be used to measure temperature. When a device is subjected to an ambient temperature increase or decrease, the distribution of charge carriers which have sufficient energy to cross the barrier increases or decreases, respectively. With an electrical bias provided across the metal contacts, a change in measured current serves as an indication of temperature change.
A pair of devices can be used in a neighborhood to provide both strain and temperature information about that neighborhood of the underlying engine part. Temperature information can be used to adjust the strain gauge information to compensate for temperature effects within the strain gauge, so that the strain gauge indication is a true indication of underlying strain. In certain engine parts, strain is cyclical, and the strain signal can be deembedded from the combined strain-temperature effects on current flowing across the Schottky barrier.
A pair of devices may also be placed together on the same stressed surface, arranged orthogonally to one another in a Poisson arrangement. Both devices are subject to the same temperature, and therefore produce the same temperature indication, e.g., the same thermionic emission current. However, due to the orthogonal arrangement of the devices, stress in the underlying engine part imparted to the semiconductor leaves of each of the devices results in different currents in each device attributable to the stress. This provides for deembedding the stress signal from the background temperature signal. The stress signal appears primarily in just one device, while the temperature signal appears in both devices. Processing of the signal can yield a corrected value indicative of the stress alone.
Furthermore, a single device can comprise three contacts, arranged in a modified Poisson arrangement in the form of a right-triangular configuration, where the center contact provides a current sink or source. The temperature signal between the center contact and each leg contact is the same. The stress signal is found primarily between just one leg and the contact. This single-chip, modified Poisson arrangement is biased by a Wheatstone bridge circuit. Stress produces current imbalance in the bridge.
The invention provides means for accurately and rapidly measuring strain in engine parts at virtually any location at which these small devices are emplaced. The devices are simple and cheap to construct, robust, and accurate over a broad range.